1. Field of the Invention
The present invention relates to a high-frequency generator circuit, which is applied to a semiconductor integrated circuit, for example.
2. Description of the Related Art
A high-frequency radio communication transmitter circuit has the following configuration. Specifically, the circuit orthogonally modulates a sine wave signal from an oscillator circuit and data signals I and Q using a mixer circuit. Then, the circuit amplifies these signals using an amplifier so that they are transmitted as a radio signal. The oscillator circuit forming the transmitter circuit, mixer circuit and amplifier circuit basically has a narrow band. For example, the mixer circuit is configured using an inductor (L) and a capacitor (C) given as a passive element. In a high-frequency radio communication transmitter circuit applied to a semiconductor integrated circuit, the passive elements needs to use a fixed value in order to realize the same operation band characteristic. For this reason, the size of these passive elements can not keep up with the scale reduction of transistors. This is a factor of causing the following problem. Namely, the area with respect to a chip of a circuit unit comprising transistors is reduced, but the area of a circuit unit comprising the passive elements is not reduced. Therefore, the area occupation ratio of the passive elements becomes large in a radio communication LSI including a digital processing circuit. As a result, this is a factor of limiting low-cost reduction in a fine process. Considering the circumstances, the following research and development has been advanced. Specifically, the transmitter circuit is not configured using mainly a conventional analog circuit, but a high-frequency signal and a radio signal are generated using a digital circuit.
For example, the following methods are given as the technique of generating a high-frequency signal and a radio signal using a digital circuit.
(1) Method of using a digital-to-analog converter circuit (DAC) (e.g., see Jpn. Pat. Appln. KOKAI Publication No. 8-204558)
(2) Method of using a digital-direct synthesizer circuit (DDS) (e.g., see Jpn. Pat. Appln. KOKAI Publication No. 2002-280838)
According to the method of using the DAC, digital data is processed by the DAC to generate a desired sine wave signal. Moreover, the method of using the DDS is basically the same as the method of using the DAC, and a sine wave signal is generated by a DAC having built-in memory and digital control circuit.
However, circuits disclosed in the methods (1) and (2) both require a high clock frequency more at least twice than a radio frequency. For example, if a high-frequency radio signal of the order of GHz or higher is generated, energy consumption of a circuit increases. Moreover, in order to obtain a high signal noise ratio of 60 dB or more, which is generally required for radio communications, ten-bit or higher resolution is required; for this reason, the circuit configuration becomes complicate. Further, circuits disclosed in the methods (1) and (2) are a voltage mode circuit; for this reason, if a power supply voltage is reduced with the scale reduction, there is a problem that resolution is reduced.
Furthermore, circuits disclosed in the methods (1) and (2) merely generates a signal of several hundred MHz at present; for this reason, these circuits is not adaptable to a high-frequency radio communication transmitter circuit, which operates at a frequency of the order of GHz or higher. Therefore, it is desired to provide a high-frequency radio communication transmitter circuit, which operates over a wide band at low voltage, and is capable of reducing the circuit area.